The tone control tests suggest that these mics are round about 1k. Even a 600ohm source means the control has less effect. 100ohm would give a -3dB point of 3.4kHz at maximum attenuation.
@Dave
That was my first thought, but they look like standard SM electrolytics, and the codes also suggest that. I'll be replacing the 0u47 ones with multilayer ceramics. They are not much bigger, more stable and much quieter.
I'm a bit concerned about the volume pot wiring, as it will have a frequency component, depending on it's position (as well as what's on the output). Near minimum the LF cutoff will be about 400Hz as only the 10ohm resistor will be relevant against the 33u cap, but at full volume it will be 4.8Hz. This could account of the apparent poor low end. So maybe it should be run as close to max as possible.
And finally a bit of bad news. My sig genny seems to have developed a fault on it's higher ranges and goes unstable when the gain is turned up
and it's only about 45 years old.I'd write a nasty letter to the manufacturer... only it was me
